From WO 2009/047041 A1 an adjustment device for varying the stroke volume of hydraulic piston machines of inclined-axis design is known. The two piston machines are arranged adjacent to one another and can be actuated together by means of a so-termed double yoke in order to vary their stroke volume. Furthermore, the piston machines are connected to one another by working lines of a closed hydraulic circuit, and each can be operated as a pump or as a motor.
The double yoke for adjusting the stroke volumes of the piston machines co-operates with the piston machines in such manner that in a first end position of the double yoke the stroke volume of the first hydraulic piston machine has a minimum value, while the stroke volume of the second hydraulic piston machine is at its maximum value.
In a second end position of the double yoke the stroke volume of the first hydraulic piston machine is at its maximum value while the stroke volume of the second hydraulic piston machine is a minimum.
To adjust the double yoke, a double-action piston-cylinder device is provided, which is functionally connected to adjustable axes of the piston machines or hydraulic machines. The piston-cylinder device is functionally connected to the axes of the hydraulic machines by a piston rod coupled to a piston.
On the rod side, the piston-cylinder device is permanently acted upon by the high pressure that acts in the area of the hydraulic machines. In accordance with the restoring force required, the size of the bottom end of the piston is made larger than the effective surface of the piston on the rod side. The adjustment forces acting in the area of the piston-cylinder device are obtained from the restoring forces acting in the area of the inclined-axis units or hydraulic machines. Thus, in the area of a hydraulic machine operated as a pump it is endeavored to reduce the acting high pressure and automatically adjust to smaller absorption volumes. In the area of a hydraulic machine operated as a motor it is also endeavored to reduce the existing high pressure and change the stroke volume in the direction of a larger absorption volume.
As a function of a power demand by the driver, the piston of the piston-cylinder device is acted upon, via a valve system, in the area of a functional surface that delimits a piston space, with pressures that correspond to a hydraulic pressure present in the area of the hydraulic machines, which act in a first positioning direction of the axes of the hydraulic machines.
By means of the valve system, which in this case comprises a position regulating valve unit and a high pressure valve control unit, besides the hydraulic adjustment of the piston-cylinder device and hence of the stroke volume of the two hydraulic machines, it is also possible to cut off the pressure and to regulate the high pressure in the hydraulic line system of the adjustment device.
To be able to provide the functionality of the high pressure regulation by means of the high pressure control valve unit, a valve slide of the high pressure control valve unit is formed with two control surfaces. The high pressure present in the area of the hydraulic machines acts on a smaller control surface in order to provide a controllable pressure cut-off function, by virtue of which the maximum system pressure in the area of the hydraulic machines can be limited essentially without loss. In the area of a larger control surface of the valve slide of the high pressure control valve unit, a control pressure produced by a proportional control pressure regulation valve is applied, by means of which the upper pressure limit of the system pressure present in the system, defined by the pressure cut-off function, can be varied as a function of the operating condition or according to need.
In practice, hydro units actuated by the adjustment device described above and comprising hydraulic machines are coupled with mechanical transmission devices and form so-termed CVTs (Continuously-Variable Transmission), which are increasingly often made with more than one gear ratio range. Between the gear ratio ranges, within which the gear ratio can in each case be varied continuously between an upper gear ratio and a lower gear ratio, the change is preferably carried out in synchronous operating conditions of the areas of the CVT involved.
During engagement of the various gear ratio ranges, the hydraulic machines are used many times and when the first gear ratio range is engaged, a first hydraulic machine is initially operated as a pump and a second hydraulic machine is operated as a motor. When a second gear ratio range, subsequent to the first gear ratio range, is engaged in the CVT, the first hydraulic machine is operated as a motor and the second hydraulic machine as a pump. If, in addition, a third gear ratio range subsequent to the second gear ratio range is provided, in this third gear ratio range the first hydraulic machine is again operated as a pump and the second hydraulic machine as a motor, as is the case when the first gear ratio range is engaged. Other ranges are possible, the hydraulic machines being used in accordance with the description above.
However, this operating mode of the hydraulic machines during the engagement of the second gear ratio range has the result that in adjustment devices known from the prior art, opening of the high pressure control valve unit, which is carried out so as to limit the pressure in the area of the hydraulic machines, increases the delivery volume of the second hydraulic machine operating as a pump instead of reducing it during the regulation of the pressure in the area of the hydraulic machines, which is undesired.
For that reason, in a transition from the first gear ratio range to a second gear ratio range, with the adjustment device described above, the functionality of the high pressure adjustment of the high pressure control valve unit has to be deactivated.
In order, despite this, to be able to avoid inadmissibly high system pressures in the system, high pressure valve units are provided in the area of which, however, during a pressure limitation, power losses again take place, which give rise to undesired heating of the system.